Search engines, directories of advertisers, and the like, e.g., available on the World Wide Web, often charge advertisers to have their listings included in a set of search results. Further, search engines and the like may charge advertisers different fees according to the position of an advertiser's listing in a set of search results. In some cases, an advertiser pays a fee for a listing included in a set of search results only when a user selects, e.g., clicks on, the listing. Such a listing may be referred to as a pay-per-click (PPC) listing. The position of a PPC listing in a set of search results, e.g., second in a set of five listings, may be determined according to a bid amount, i.e., an amount that an advertiser is willing to pay upon a user selection of the listing.
From the standpoint of being clicked on by a user, it is generally advantageous for a listing to be placed in a higher position, e.g., first as opposed to fourth in a set of five listings, because users often follow a tendency to click on listings that are placed in higher positions. Nonetheless, regardless of where they are placed in a set of search results, some listings may be more or less likely to be clicked on by a user than other listings. For example, two listings that may be placed in a third position in a set of five search results may have entirely different likelihoods of being clicked on by a user if so placed.
Where a search engine operator provides listings such as PPC listings, listings that are more likely to be clicked on by a user are more likely to generate revenue for the search engine operator than other listings. However, average click-through rates often vary dramatically among listings in different categories, e.g., categories of business, and among listings associated with different geographic locations. As a result, using the same average click-through rate for all categories and geographic locations may not foster accurate comparisons of click-through rates associated with different categories and/or geographic locations.